This invention relates generally to the field of precision guidance systems, and more particularly to a system for use in guiding a movable worktable on a machine tool in which bearing members are biased into engagement with a guide rail to accommodate for imprecisions in the components of the guidance system.
A variety of machine tools have been designed in which the workpiece to be machined is mounted on a worktable which is movable relative to the cutting tool which is to perform the machining operation. One of these types of machine tools are drilling machines which have been designed to drill holes in printed circuit boards. Printed circuit boards have a large number of holes which must be drilled at precise locations to enable mounting of electrical components on the boards. Computer controlled drilling machines have been developed to permit mass production of circuit boards at high rates. Typically, these drilling machines have a worktable mounted on a base and movable along both axes of a horizontal plane. The printed circuit boards, or workpieces, are mounted in stacks on the worktable. Movement of the worktable in the horizontal plane positions the circuit boards beneath a drill spindle at the precise location where holes are desired. Drilling operations are achieved through vertical travel of the drill spindle.
Since the diameter of the holes drilled in the circuit boards can be extremely small, on the order of the size of human hairs, it is imperative that movement of the worktable be precisely controlled. In prior systems, such as that illustrated in FIG. 4, a worktable 10 is guided along one axis by a guide rail 12 having two straight, parallel, vertically oriented side surfaces 14, 16. The worktable 10 straddles the sides 14, 16 of the guide rail 12 as it moves parallel to the side surfaces 14, 16. The worktable 10 includes air bearing shoes 18, 20 which engage each side of the guide rail 12 to minimize friction. Although not shown in FIG. 4, the left-hand bearing shoe 20 has a ball and screw attached in a similar manner to the ball 26 and the screw 24 shown in the right-hand side of FIG. 4.
To reduce manufacturing costs, only one of the guide rail side surfaces 16 is machined to be truly linear and planar. The second side surface 14, although still machined to close tolerances, may have slight irregularities or lack true parallelism with the first surface 16. To ensure that the worktable 10 moves parallel to the first surface 16, the bearing 18 which engages the second surface 14 is biased by a Belleville washer 22 which deflects to allow displacement of the bearing 18 toward or away from the second surface 14, thus accounting for depressions or raised portions of the second surface 14, respectively.
Several drawbacks are prevalent with these prior systems. First, in order to preload the washer 22 under the proper amount of compressive force to maintain the desired running clearance between the air bearing shoes 18, 20 and the guide rail surfaces 14, 16, a screw 24 must be threaded through the worktable 10 and driven against a ball 26 nesting within the washer 22. This screw 24 transmits the preloading force to the worktable 10 itself and may cause distortions or deflections in the worktable 10. Second, the biasing force provided by the washer 22 is somewhat unpredictable due to the frictional engagement between the washer 22 and the air bearing shoe 18 as the washer 22 deflects. As forces on the washer 22 vary, hysteresis effects are induced in the washer 22, further limiting the repeatability of the performance of the washer 22 as a biasing member.
Similar problems have been encountered with the guidance system disclosed in Smith U.S. Pat. No. 3,578,827, which also permits spring preload forces to be transmitted to the precision components of the guidance system.
Thus, a need exists for a guidance system in which preload forces on the biasing member are not transmitted to the movable member being guided, and in which the biasing force applied is repeatable and predictable.